The dirt-bed has been described above in its most simple form, but in some sections the appearances are more complicated. The forest of the dirt-bed was not everywhere the first vegetation which grew in this region. Two other beds of carbonaceous clay, one of them containing _Cycadeae_, in an upright position, have been found below it, and one above it[234-A], which implies other oscillations in the level of the same ground, and its alternate occupation by land and water more than once. _Table showing the changes of medium in which the strata were formed, from the Lower Greensand to the Portland Stone inclusive, in the south-east of England._ 1. Marine Lower greensand. 2. Freshwater Weald clay. 3. Freshwater } Brackish } Hastings sand. Freshwater } 4. Freshwater Upper Purbeck. 5. Freshwater } Brackish } Marine } Brackish } Middle Purbeck. Marine } Freshwater } Marine } 6. Freshwater } Brackish } Land } Freshwater } Land (dirt-bed) } Lower Purbeck. Freshwater } Land } Freshwater } Land } Freshwater } 7. Marine Portland stone. The annexed tabular view will enable the reader to take in at a glance the successive changes from sea to river, and from river to sea, or from these again to a state of land, which have occurred in this part of England between the Cretaceous and Oolitic periods. That there have been at least four changes in the species of testacea during the deposition of the Wealden, seems to follow from the observations recently made by Professor E. Forbes, so that, should we hereafter find the signs of many more alternate occupations of the same area by different elements, it is no more than we might expect. Even during a small part of a zoological period, not sufficient to allow time for many species to die out, we find that the same area has been laid dry, and then submerged, and then again laid dry, as in the deltas of the Po and Ganges, the history of which has been brought to light by Artesian borings.[235-A] We also know that similar revolutions have occurred within the present century (1819) in the delta of the Indus in Cutch[235-B], where land has been laid permanently under the waters both of the river and sea, without its soil or shrubs having been swept away. Even, independently of any vertical movements of the ground, we see in the principal deltas, such as that of the Mississippi, that the sea extends its salt waters annually for many months over considerable spaces, which, at other seasons, are occupied by the river during its inundations.